This invention relates to an electrolyte for electrolytic capacitors and more particularly for an electrolyte for high voltage aluminum electrolytic capacitors.
Electrolytic capacitors are well known for use in a variety of electronic equipment used for data processing, communication, entertainment and power electronics. Generally, these capacitors are made of a capacitor element which is comprised of anode and cathode foils separated by layer(s) of spacer paper. The electrode foils are generally made of aluminum which has undergone etching to increase surface area. The anode foil has undergone an additional formation process to grow an anodic barrier oxide layer that serves as the capacitor dielectric. Short lengths of wire or strips of narrow sheet stock, mechanically bonded to the lengths of anode and cathode foil in the capacitor element, are used to make electrical connection to the external circuit. The capacitor element is placed into a protective case. Depending upon the capacitor design and size, electrolyte is impregnated into the capacitor element either before or after it is placed in the case. Details of capacitor design and assembly procedures vary with manufacturer. Generally, if wire connectors are used, they pass through an elastomer seal in the top of the case. If narrow sheet strip is used for electrical connection, it is welded or riveted to terminals embedded in a rigid polymer cover. The cover is then tightly sealed to the case.
Current capacitor technology teaches use of electrolytes containing ethylene glycol and a co-solvent for high voltage use. The solutes used in these electrolytes are salts of boric acid or dicarboxylic acids, or mixtures thereof.
U.S. Pat. No. 3,487,270 teaches the use of polyalkylene glycols as a co-solvent for high voltage electrolytes and particularly a polyethylene glycol having a molecular weight of greater than 2000 as a co-solvent in a glycol borate electrolyte. Japanese Pat. No. JP62268121 [87268121] teaches the use of a polyethylene glycol having a molecular weight of 400 in a high voltage electrolyte and Japanese Pat. No. JP02312218 [90312218] teaches the use of polyethylenepropylene glycol having a molecular weight of 1000 to 4000. However, these co-solvents all cause the electrolyte to have a high resistance at low temperatures, e.g. -25.degree. C. The result is that the capacitor equivalent series resistance (ESR) is too high and the capacitance is too low for the capacitor to be useful in an electronic circuit operated at low temperature.
Moreover, Japanese Pat. No. JP0562862 [9362862] teaches the use of 2,2-dimethy-1,3-propanediol; 2-butyl-2-ethyl-1,3-propanediol; and 2,3-dimethyl-2,3-butanediol co-solvents in a capacitor electrolyte, but these solvents have high melting points and cause poor low temperature properties.